High school and college athletes are enrolling in strenuous fitness programs yet the long-term effect of such intense physical activity is not known. Exercise is very effective at inducing fat-loss which will be reflected by decreasing levels of leptin, a hormone that is produced by fat tissue. The total amount of body fat can affect the menstrual cycle in girls and very young women. This study is a research project that will determine leptin levels in girls and very young women in various states of physical fitness. The many benefits of exercise could be out-weighed by it's negative effect on a young women's menstrual cycle. The reduction of total body fat may be responsible for this effect of exercise even if there is no significant decrease in body weight. It is important to better understand how exercise affects women order to devise better ways to monitor the intensity of training. We plan to study a total of 80 adolescent girls of normal weight between 16 years and 20 years. There will be forty sedentary girls with normal cycles, 20 athletes with normal cycles and 20 athletes with menstrual irregularities. Twenty sedentary girls will be studied before and after four months of intensive training aiming at improving fitness while preserving a normal weight. Measurements of leptin, a hormone produced by fat tissue, may help in monitoring the minimal amount of body fat that is necessary for a normal menstrual cycle. Such information could help in designing effective and safe athletic programs for young women aiming at preserving the menstrual cycle and their ability to bear children.